This invention relates to a fuel supply control method for multicylinder internal combustion engines, and more particularly to a control method of this kind which is adapted to control the sequence of fuel injections into engine cylinders at high speed operation of the engine in particular.
A fuel supply control method for multicylinder internal combustion engines has generally been employed, which comprises successively calculating the quantity of fuel to be supplied to the engine to values dependent upon operating conditions of the engine, in synchronism with generation of pulses of a crank angle signal sequentially generated at predetermined crank angles of the engine, and successively supplying the calculated quantities of fuel to the engine cylinders in predetermined sequence.
According to such conventional sequential fuel supply method, when the engine speed is low, the time interval between adjacent pulses of the crank angle signal sequentially generated is larger than the maximum possible time required for calculating the fuel quantity such that even if the calculation is started upon generation of each pulse of the crank angle signal and the supply or injection of the calculated quantity of fuel is started immediately after completion of the calculation, almost all the fuel thus supplied is sucked into the corresponding cylinder. Thus, in a low engine speed region, a required air-fuel ratio and accordingly required driveability of the engine can be achieved with the conventional method.
However, when the engine speed is so high that the time interval between adjacent pulses of the crank angle signal sequentially generated is very close to the maximum possible calculating time, the injection timing will be too late relative to the timing of the suction stroke of the corresponding cylinder if the fuel injection is started after completion of the calculation, failing to positively achieve a required air-fuel ratio and accordingly required engine driveability.
To overcome this disadvantage, there have been proposed a method of decreasing the frequency of calculation of the fuel quantity, for instance, effecting the calculation one time per several pulses of the crank angle signal sequentially generated during high speed operation of the engine, and then injecting the calculated quantity of fuel into two cylinders in a group of two groups at the same time or into all the four cylinders at the same time in the case of a four cylinder engine (Japanese Patent Publications Nos. 47-38657 and 49-45652), and a method of holding fuel injection immediately after completion of the calculation, and starting the fuel injection of the calculated quantity into the corresponding cylinder upon generation of a crank signal pulse after execution of one cycle of the cylinder, thus continually effecting sequential injection in a high engine speed region as in a low engine speed region.
However, according to the former proposed method, there can occur a large time lag, particularly in simultaneous injection into all the four cylinders, between the completion of the calculation and the start of fuel injection, thus suffering from low responsiveness to changes in the operating condition of the engine and accordingly degraded engine driveability. On the other hand, according to the latter proposed method, upon changeover of the injection manner at transition from the low engine speed region to the high engine speed region, there can occur a large change in the injection timing relative to the suction stroke of the cylinder(s), leading to fluctuations in the air/fuel ratio and accordingly degraded engine driveability. That is, referring to FIG. 1 showing the latter proposed method, if in the low engine speed region a calculation CAL is started upon generation of a pulse of the crank angle signal corresponding to a cylinder #1 [(b) of FIG. 1] and injection of the calculated quantity of fuel is started immediately upon completion of the calculation, the injection starting timing will become delayed relative to the cylinder suction stroke as the engine speed becomes higher [(c) of FIG. 1], resulting in that only part Q1 of the injection fuel is actually sucked into the cylinder #1, while the remainder of the injected fuel remains within the intake pipe and is sucked into the next cylinder during the suction stroke thereof. That is, a fuel quantity Q'2 remaining in the intake pipe at the suction stroke of a preceding cylinder (nearly equal to the remaining fuel quantity Q2) is sucked into a present cylinder together with a fuel quantity Q1 just injected. Therefore, so long as the engine is operating at a constant speed, almost the same fuel quantity Q is sucked into each of the cylinders, and thus neither air-fuel ratio fluctuation nor degradation of the engine driveability takes place. On the other hand, upon transition from the low engine speed region to the high engine speed region, as shown at (d) in FIG. 1, fuel injection into each cylinder is started upon generation of a crank angle signal pulse T1 after execution of one cycle of the cylinder from the completion of the calculation for the cylinder, that is, the injection timing is largely retarded relative to the starting time of an immediately preceding fuel injection as shown at (c) in FIG. 1. In addition, all of a fuel quantity Q obtained by the immediately preceding calculation CAL and now injected upon generation of the pulse T1 and a fuel quantity Q2 remaining unsucked in the intake pipe at the immediately preceding injection are sucked together into the cylinder #1, suddenly overriching the air-fuel ratio and accordingly causing a sudden change in the engine speed, i.e. operating shock, or temporary degradation in the driveability.